1. Field of the Invention
The present invention relates to a method for producing a Group III nitride semiconductor having reduced threading dislocation and good crystallinity.
2. Background Art
Conventionally, a method has been known, in which through Metal Organic Chemical Vapor Deposition (hereinafter referred to as “MOCVD”), a low temperature buffer layer is formed on a sapphire substrate, and GaN is grown on the buffer layer.
For example, in Japanese Patent Application Laid-Open (kokai) No. 2005-19872, after a sapphire substrate is heat-treated at 1,135° C. to clean the surface thereof, with the substrate temperature lowered to 515° C., a GaN buffer layer having a thickness of 20 nm is formed, and with the substrate temperature increased to 1,075° C., fine crystals of GaN are formed on the sapphire substrate. Subsequently, with the substrate temperature maintained at 1,075° C. and the concentration of hydrogen higher than that of nitrogen in the carrier gas, GaN is facet grown using the fine crystals of GaN as nuclei. Then, the substrate temperature is lowered to 1,005° C., and the concentration of nitrogen is made higher than that of hydrogen in the carrier gas to facilitate growth in a lateral direction, and GaN is grown so as to fill gaps among facets. Thus, GaN having reduced threading dislocation density is obtained.
In EXAMPLE 3 of Japanese Patent Application Laid-Open (kokai) No. 2005-183524, after a sapphire substrate is thermally cleaned at 1,200° C., with the substrate temperature at 1,200° C., AlN is epitaxially grown to form a single crystal underlying layer 102 having a thickness of 0.7 μm. Subsequently, with the substrate temperature lowered to 1,150° C., an AlGaN layer 103 is epitaxially grown so as to have a thickness of 100 nm or less, and annealing is performed with the substrate temperature maintained at 1,350° C. for 10 minutes. Then, with the substrate temperature lowered to 1,150° C., an AlGaN layer 104 is further grown. Thus, the threading dislocation density of the AlGaN layer is reduced.
However, in the method of Japanese Patent Application Laid-Open (kokai) No. 2005-19872, after forming a low temperature buffer layer of ultrathin GaN having a thickness of 20 nm at a low temperature, with the temperature increased to a temperature at which GaN can grow, fine crystals of GaN are formed, and then GaN is facet grown. The fine crystals of GaN of the low temperature buffer layer formed at a low temperature are decomposed and evaporated again in the process of temperature increase to the GaN growth temperature. Therefore, after the formation of the low temperature buffer layer, the temperature of the substrate cannot be increased more than a temperature at which GaN can grow. This results in insufficient formation of fine crystal nuclei, and the crystal nuclei cannot grow large. Therefore, the density at the starting point of threading dislocation is still high.
In the method of Japanese Patent Application Laid-Open (kokai) No. 2005-183524, the underlying layer 12 on the base material 11 is epitaxially grown in a thickness of 0.7 μm, and is therefore a single crystal. Moreover, the AlGaN layer 103 is epitaxially grown on the single crystal underlying layer 102, and is therefore a single crystal. Annealing at a stage that the AlGaN layer 103 has been formed facilitates movement of dislocation in the AlGaN layer 103, and thereby reducing the dislocation density (paragraph 0032).
Therefore, Japanese Patent Application Laid-Open (kokai) No. 2005-183524 neither increases the size of the crystal nuclei in the buffer layer in a polycrystal, amorphous or polycrystal/amorphous mixed state, nor inhibits the formation of threading dislocation in the semiconductor layer to be grown.
Japanese Patent Application Laid-Open (kokai) No. 2005-19872 relates to a heat treatment for obtaining the crystal nuclei for facet growth, and does not reduce the density at the starting point of threading dislocation.
The present inventors found for the first time that when GaN is grown after high-temperature heat treatment of AlN buffer layer formed at a low temperature, irregularities or roughness on the surface of GaN become large. They clarified for the first time that the cause of the above problem is that the Al contained in the buffer layer is oxidized by high-temperature heat treatment, and Al oxide is formed, thereby the growth surface of GaN grown on the Al oxide has an N-polarity. N-polar GaN has large surface roughness, and more impurities are incorporated therein. Therefore, it is not suitable for the device. When an oxide is formed, a new crystal defect (dislocation or deposition defect) is more likely to occur on the oxide, which reduces the crystal quality of the semiconductor to be grown.